1. Technical Field
This invention relates to methods and devices to confirm the presence or absence of a chemical agent after a monitor for the detection of that agent alarms.
2. Description of Related Art
It is becoming a common practice both in military and industrial applications to continuously monitor the atmosphere to detect and to warn of the presence of a toxic chemical agent or other chemical compound of environmental concern. Monitoring is ordinarily accomplished using a near-real-time (NRT) monitor alarm system that is designed to detect sub time weighted average (TWA) concentrations of the chemical agent or compound of interest. As a result, such systems operate at the limits of sensitivity and selectivity so as to provide the maximum protection to exposed workers and the environment. An undesirable consequence of operating a detection system at its sensitivity and selectivity limits is the inevitable production of false positive alarms that can result in large increases in operating costs.
It is desirable to quickly confirm the presence or absence of the chemical agent when a NRT monitor sounds an alarm. Confirmation of the NRT analysis requires a second analysis of the same atmosphere that generated the original alarm and also requires that the confirmation technique used have at least equivalent, and preferably better, sensitivity and selectivity than does the NRT monitor. To achieve that end, sufficient quantities of the original air sample must be continually collected to allow analytical confirmation of any single cycle event that triggers an alarm. Complicating the problem is the need to minimize the cycle time of the NRT monitor. Cycle time is that period between taking a particular sample and reporting the results of the analysis of that sample, and typically ranges from about three to fifteen minutes depending upon the application.
NRT confirmation techniques in current use typically employ a depot area air monitoring system (DAAMS tube) for the collection of confirmation samples. The DAAMS system uses solid sorbents packed within a glass or stainless steel tube to collect the sample. The sample is then thermally desorbed into a gas chromatograph for separation and detection. Use of the DAAMS system is advantageous in that it allows the trapping and concentration of a large volume sample in a single sampling tube without the use of trapping solvents that would otherwise dilute the sample. The DAAMS tubes are reusable and generate virtually no waste. Major disadvantages of the DAAMS system are that it requires unique and proprietary automatic thermal desorption equipment for sample introduction and that the entire sample is consumed during the analysis, thus precluding multiple or repeat analysis of a sample.
Physical limitations dictate how the confirmation of an event can be accomplished. The TWA concentrations for most chemical agents require that the NRT monitor operate at its maximum achievable sensitivity and selectivity and its minimum cycle time. Consequently, there are a number of parameters that affect the efficacy of NRT confirmation monitoring. Among those parameters are the sampling rate and the kind or type of sampling that is conducted. The sampling rate for a NRT confirmation system is dependent upon the sensitivity of the method used to analyze the confirmation sample. Sensitivity of the confirmation analysis is typically no better than is that of the NRT monitor. Hence, the sampling rate for the confirmation sampler needs to be as high if not higher than the sampling rate for the NRT sampler.
There are currently two approaches to confirmation sampling that differ in kind or type; continuous and on-demand sampling. In continuous sampling, a DAAMS tube is placed at the same location as is the NRT monitor and the tube collects sample as the NRT monitor operates. An advantage to that approach is that when the NRT monitor signals an alarm the atmosphere which generated the alarm has been concurrently sampled and any chemical agent present has been captured on the sorbent loaded in the DAAMS tube. Disadvantages are that the confirmation sampling has been conducted over multiple NRT monitor cycles, and compounds captured by the DAAMS tube often include contaminants and interferents in addition to the chemical agent. Another disadvantage to continuous sampling is that it is cumulative. If chemical agents are present in the atmosphere in such low levels as to be undetectable by the NRT monitor they would accumulate on the DAAMS tube. Over time, the level of agent captured by the DAAMS tube would build up to a point where it would be difficult or impossible to associate the agent seen by confirmation sampling with an actual alarm event. Further, some chemical agents degrade rapidly after their release to the environment, and those agents are generally not amenable to a continuous sampling approach.
In on-demand sampling, the NRT monitor is used to control the operation of a confirmation sampler placed at the same location. When the NRT monitor generates an alarm, it also produces a signal that turns on, or energizes, the confirmation sampler. In current practice, the confirmation sampler employs three DAAMS tubes. The confirmation sampler, upon receiving an alarm signal from the NRT monitor, draws air through the first DAAMS tube for a pre-set time period, typically about fifteen minutes. If the NRT monitor is still in alarm status at the end of the first sampling period, the confirmation sampler sequences to the second DAAMS tube. Otherwise, the confirmation sampler waits for the next alarm event that is captured with the next tube in the sequence. That mode of operation continues until all three DAAMS tubes have been used or the tubes have been collected and the sampler reset.
On-demand sampling also has unique advantages and disadvantages. One advantage is the near elimination of contaminant or interferent buildup on the tube as well as the accumulation of chemical agent that is present in the atmosphere at levels below the detectability limit of the NRT monitor. In addition, the pump used to draw sample through the DAAMS tubes operates only when an alarm event is suspected, thus considerably increasing pump life. Logistical difficulties and concerns associated with changing out DAAMS tubes in the field are reduced as well. A primary disadvantage to on-demand sampling that the atmosphere which causes the NRT monitor to trigger an alarm is not sampled by the confirmation sampler. Rather, the sampled atmosphere is that one present a short time, a few minutes, after the triggering event. That circumstance opens the possibility of being unable to confirm a transient, or single cycle, event.
It is apparent that a confirmation sampling system combining the advantages of both currently used approaches while reducing or eliminating their disadvantages would be a significant advance in the art.